This invention relates to probing apparatus in which a probe electrode is inserted in plasma to pick up various kinds of information of the plasma, and more particularly, to apparatus for detecting degree of contamination on probe surface which is contaminated by ionized materials and so on.
In the field of plasma chemistry, technologies such as property modification and purification of solid surface, synthesis and decomposition of materials and the like have been developed by use of the plasma. In apparatus for such plasma processings, it is very important to accurately pick up information such as ion distribution, electron density and electron temperature in the plasma in order to control plasma conditions with high precision. Probing methods for measuring the various parameters, in which a single probe or a plurality of probes are inserted in the plasma, have been already known. However, since the probes in the plasma are always exposed to an ionized gas, it is gradually contaminated with ionized materials, sputtered materials and so on. The measurement can not be carried out with high accuracy and high precision if the probe surface is contaminated.
Some methods for detecting the degree of the contamination on the surface of the electrode, which will be described hereinafter, have been used.
FIG. 1 shows a circuit diagram for detecting the contamination on the probe surface according to a prior art method. In single probe methods, an electrode P.sub.1 is used as the probe electrode and an electrode P.sub.2 is used as a reference electrode. In double probe methods, the electrodes P.sub.1 and P.sub.2, which have the same faculties, are inserted in the plasma. A sweep voltage from a sweep voltage generator 1 is applied between the probe electrodes and then a probe current I.sub.p is detected by a current detecting circuit 2, so that a voltage-current characteristic curve is drawn by a two dimensional display 3 such as a X-Y recorder or an oscilloscope. Typical curves of voltage-current characteristics are shown in FIGS. 2 and 3. FIG. 2 corresponds to the single probe method and FIG. 3 to the double probe method. When the probe surface is not contaminated, the voltage-current characteristic is shown by the curve A or A' in either case of increasing or decreasing of the sweep voltage. On the other hand, when the probe surface is contaminated, a different characteristic is obtained, as indicated by the curves B and C or B' and C'. The curve B and B' corresponds to a characteristic during increasing of the sweep voltage and the curve C and C' to a characteristic during decreasing of the sweep voltage. In other words, when the probe surface is clean, the same characteristic is obtained in either case of increasing or decreasing of the sweep voltage, but when the probe is contaminated, a hysteresis loop is appeared on the probe voltage-current characteristic. Therefore, the contamination of the probe surface have been checked based on whether there is a hysteresis loop on the probe characteristic.
In general, when the probe surface is contaminated, the internal parameters of the plasma, specifically, electron temperature may be overestimated, so that judgement of plasma voltage or estimation of electron density becomes uncertain. Therefore, in the probing methods, cleanness of the probe surface should be secured on the measurements of the inner parameters of the plasma. In order to clean the probe surface, the probe is heated except during the measurements on the internal parameters and the cleanness of the probe surface should be confirmed by the method for detecting the contamination before the measurement of the internal parameters. But since the prior art methods for detecting the contamination spend a few minutes or more when the oscilloscope is used or a few seconds or more when the X-Y recorder is used, the contamination of the probe surface may progress during the measurement of the contamination. In order to detect the contamination at a high-speed based on the prior art methods, expensive apparatus and troublesome manipulations are required.
In brief, an apparatus for rapidly detecting the contamination on the probe surface is required.